This invention relates generally to the field of high density electronic circuitry, and more specifically to high density electronic circuitry adapted for mounting devices such as integrated circuits (ICs).
Historically, ICs communicated through input/output (I/O) pads located on the periphery of the IC. Each IC mounted on its own substrate. A circuit board then connected multiple ICs to form a system. Improvements to IC technology and increased system performance demands have made these historical practices impractical. Larger, more dense ICs eventually run out of periphery space for I/O pads. The routing of signals to periphery pads, then across a substrate to a circuit, then across the circuit to another IC adds to the complexity, capacitance, and inductance of the system. This increases the system cost and reduces the system performance.
ICs can be manufactured with I/O pads on the IC's face instead of just on its periphery. ICs can also mount face down on substrates so that the I/O pads can connect to the substrate. This technique, often called flip-chip mounting, can overcome the I/O density limitation of peripheral I/O pads. See, e.g., "Implementation of flip chip and chip scale technology", Sematech Joint Industry Standard J-STD-012. Direct placement of ICs to the substrate also offers other electrical and thermal advantages.
The direct placement of ICs onto the substrate can cause many problems, however. Differential expansion of the IC and the substrate can destroy the substrate-IC connection. The substrate-IC connection also depends on the planarity of the IC attachment points. The substrate must also provide high density interconnection circuitry to connect multiple ICs into one system.
Substrates have been made from the same material and by the same processes as the ICs. Differential expansion is not a problem, and high density interconnects are possible. See, e.g., Bartelink, U.S. Pat. No. 5,189,505 and U.S. Pat. No. 5,077,598. Substrates made this way can be very expensive, however, making them impractical for many applications.
Substrates made of materials different from the ICs can be less expensive, but must accommodate differential expansion. Some approaches mount ICs to the substrate with adhesive; this addresses the differential expansion problem but can cause other severe problems if the system later requires repair. See, e.g., Bachler, U.S. Pat. No. 5,147,208. Other approaches build on traditional printed wiring board technologies, but encounter circuit density limitations due to mechanical registration problems for through-hole vias and can suffer from differential expansion problems. See, e.g., Marrs, U.S. Pat. No. 5,355,283. Other approaches address the differential expansion problem by using raised interfacing features to provide stress relief. These all encounter limits on circuit density, feature height, alignment, or attachment point planarity. See, e.g., Bachler, U.S. Pat. No. 5,147,208; Higgins, U.S. Pat. No. 5,434,452; Farnworth, U.S. Pat. No. 5,487,999; Love et al., U.S. Pat. No. 5,334,804.
There is a need for improved electronic circuits and methods for making them. The improved methods must allow direct mounting of devices such as ICs and must accommodate differential expansion of the device and the circuit substrate. The circuits must provide high density interconnections among the devices, and the methods for making the circuits must allow inexpensive manufacture.